Looking for signs of "stress, fear and deception" among the hundreds of passengers shuffling past him at Orlando International Airport one day last month, security screener Edgar Medina immediately focused on four casually dressed men trying to catch a flight to Minneapolis.

One of the men, in particular, was giving obvious signs of trying to hide something, Medina said. After obtaining the passengers' ID cards and boarding passes, the Transportation Security Administration officer quickly determined the men were illegal immigrants traveling with fake Florida driver's licenses. They were detained.

"It wasn't that unusual," Medina said. "We see more and more of that stuff down here. Every day, that is what I'm looking for."

The otherwise mundane arrests Aug. 13 illustrated an increasingly popular tactic in the government's effort to fight terrorism: detecting lawbreakers or potential terrorists by their behavior. The TSA has embraced the strategy, training 600 of its screeners, including Medina, in detection techniques. Such screeners patrol the Washington region's three airports, and by year's end, 1,000 screeners at more than 40 airports will be trained.

The TSA also plans to train screeners in the art of observing slight facial movements that indicate a person is lying.

Although civil libertarians and top Democrats in Congress say the techniques raise serious questions about privacy rights and racial and ethnic profiling, TSA officials say the behavior-detection officers may play a more important role in thwarting terrorist attacks than traditional screening techniques.

The teams have referred more than 40,000 people for extra screening since January 2006. Of those passengers, nearly 300 were arrested on charges including carrying concealed weapons and drug trafficking. TSA officials will not say whether the screeners have helped nab potential terrorists, but they say terrorists and other lawbreakers exhibit the same behavioral clues.

"In this kind of environment, you can't be sure they are going to come to the checkpoint with a prohibited item, per se," said Kip Hawley, the TSA's administrator. "Unless you do something more than that, you are going to miss the next attack. A behavior-detection officer will detect somebody no matter what the weapon is."

The TSA's teams are the most publicly acknowledged effort by the government or the private sector to come up with strategies and technology to detect lawbreakers or terrorists before they commit a crime. Other technologies under development or being deployed include machines that detect stress in voices and software that scans video images to match the faces of passengers with those of known terrorists.

The government is testing other technology that can see through clothing with Superman-like vision or can help determine whether somebody might be carrying an explosives-laden vest by analyzing electromagnetic waves. Some of that technology, including back-scatter X-ray devices, has already been deployed at some airports and in mass-transit terminals.

TSA officials acknowledge that those technologies are years from deployment and may not be as flexible as their behavior-detection officers, whom they can post outside airports, in terminals or train stations, or at checkpoints.

The behavior-detection program works like this: On a recent afternoon, two specially trained officers -- they always work in teams -- stood by a checkpoint at Dulles International Airport while another team of two roamed the terminal. The officers watched for anyone who seemed nervous, out of place or was acting suspiciously.

The TSA won't publicly disclose what behavior screeners are looking for. However, screeners, former screeners and consultants say the officers are looking for people traveling without bags, sweating and constantly checking out every person passing by, especially those with badges and guns. People who avoid eye contact or veer away when police approach also draw their attention.

When deciding whether to target a passenger, TSA screeners generally do not rely on one trait but a combination of behaviors.

On the recent afternoon, the two roaming officers were keeping an eye on a passenger who had gone through and then exited security. The man was sitting on a concrete barrier at the passenger drop-off area. He didn't have bags, smoked a cigarette, seemed to be talking to himself and kept standing up and sitting down. Eventually, when he returned through the checkpoint, officers pulled him aside and questioned him. They said they found nothing suspicious and let him go.

To become a behavior-detection officer, screeners undergo four days of classroom training and three days of supervised on-the-job work.

A new tool in their arsenal is the ability to determine when the slightest facial movement is masking a lie. All of the TSA's behavior-detection officers and the agency's 1,000 inspectors, who also work at airports, will be trained in the technique.

David Matsumoto, research director for the Ekman Group, which conducts the TSA "micro-facial expression" training, said that micro-expressions are signs of concealed emotions and "are indications that the travelers have an emotional state that they don't want anyone else to know about."

The expressions often last less than 1/15 of a second, he said.

"When you're not trained to see them, when you blink, you'll miss them," Matsumoto said. "Even if you don't blink, they're so fast people don't realize what is happening."

The TSA's growing reliance on detecting behavior and the close study of passengers' expressions concerns civil liberties groups and members of Congress.

"The problem is behavioral characteristics will be found where you look for them," said John Reinstein, legal director of the American Civil Liberties Union of Massachusetts, which is suing the Massachusetts State Police over an incident in which an officer trained in behavior detection detained a passenger at the airport in 2003.

"The fact remains that Muslims and people of Middle Eastern descent are perceived to be of particular threat," he said. "So it is highly likely that those are the people whose behaviors will be more highly scrutinized. There is still the danger that [the technique] will be used in a racially discriminatory manner."

Rep. Bennie Thompson (D-Miss.), chairman of the House Homeland Security Committee, said the program's aggressive expansion caused him concern. He plans to hold hearings on the issue in the next few months, he said.

"We have to be careful in using this so we don't single out people who look different than us," Thompson said. "When we get into something that is approaching behavior, we have to be very careful that we don't stereotype people because of their dress or their race. And we have to understand and protect the civil liberties and civil rights of people in this country."

TSA officials say they have received no complaints from passengers about profiling or privacy violations connected to the behavior-detection effort.

"We spend a substantial portion of our training going over why everyone knows racial profiling is illegal," said Carl Maccario, a TSA program analyst who coordinates the detection effort. "As a security tool, it is also ineffective. If you are racially profiling, the real terrorist is going to slip past you. This is actually an antidote to racial profiling, because officers have to articulate exactly what made them suspicious."